In the 3rd generation partnership project (3GPP), the uplink synchronization technology in the long term evolution (Long Term Evolution, hereafter referred to as LTE) system includes two parts as follows: In one part, a user equipment (User Equipment, hereafter referred to as UE) needs to obtain uplink synchronization again after losing the uplink synchronization. Specifically, the UE sends a random access channel (Random Access Channel, hereafter referred to as RACH) preamble (preamble) to a base station; the base station obtains a time advance (Timing Advance, hereafter referred to as TA) of initialized uplink synchronization of the UE uplink carrier according to the received RACH preamble; then, the base station sends the TA to the UE; and the UE obtains an uplink sending time according to the TA. As a result, the initialized uplink synchronization is implemented. In the other part, the UE needs to maintain the uplink synchronization in an uplink synchronization state. Specifically, the UE sends a reference symbol (Reference Symbol, hereafter referred to as RS) to the base station, and the RS includes sounding reference symbol (Sounding Reference Symbol, hereafter referred to as SRS) and demodulation reference symbol (Demodulation Reference Symbol, hereafter referred to as DMRS); the base station obtains, according to the received RS, the TA that is needed by the UE to maintain the uplink synchronization; then, the base station sends the TA to the UE; and the UE obtains an uplink sending time according to the TA. As a result, the uplink synchronization is maintained.
In the case of carrier aggregation of multiple uplink members, hetero-frequency switchover in the same system, hetero-frequency switchover in different systems, or loss of synchronization, the UE needs to perform a RACH access process. The case of the carrier aggregation of multiple uplink members is taken as an example. During an initialized access process, the UE can only perform the RACH access process on a certain uplink member carrier (for example, carrier f1) to obtain the TA of carrier f1. Before the UE needs to send data on another uplink member carrier (for example, carrier f2), the UE needs to obtain the uplink synchronization of the carrier f2 first, and also needs to perform the RACH access process on the carrier f2. That is to say, the UE needs to perform the RACH access process on each uplink member carrier, so as to obtain the initialized uplink synchronization In the same way, the UE also needs to send the RS on N different uplink member carriers to maintain the uplink synchronization, then, the base station obtains N TAs, and needs to maintain the N TAs, and then, sends the N TAs to the UE.
In the case of the forgoing carrier aggregation, when the RACH preamble is sent once, it takes at least 8 ms for the UE to obtain the TA; when the RACH preamble is repeatedly sent for n times, it takes at least 8 n ms for the UE to obtain the n TAs. For some services which are sensitive to a delay, the delay in this processing method is too long, and occupies certain network resources, reducing the performance of a system. Meanwhile, in an uplink synchronization maintenance stage, the base station needs to maintain multiple TAs, and send the multiple TAs to the UE, which is a waste of network resources. In the same way, in the case of the hetero-frequency switchover in the same system, hetero-frequency switchover in different systems, or loss of synchronization, when the UE performs a RACH access process, the problem that the delay is too long and the network resources are wasted may exist.